This invention relates to a method for electrodialytically recovering valuable metal-plating chemicals from dilute aqueous solution, such as contaminated rinse water, for use in an aqueous electroplating bath. More particularly, this invention relates to an electrodialytic recovery method adapted to be carried out within an electroplating bath concurrent with metal-plating operations and utilizing a portion of the plating current to extract chemicals from dilute solution directly into the bath.
A metal plate is electrodeposited onto a workpiece by immersing the workpiece in an aqueous bath having a relatively concentrated ionic metal species dissolved therein and cathodically biasing the workpiece to reduce and plate the metal. The plated workpiece is removed from the bath and cleansed with clean water to rinse away residual plating solution dragged out with it. The rinse water thereby becomes contaminated by low concentrations of valuable plating chemicals.
Electrodialysis has been emmployed to extract the chemicals from the rinse water for return to the bath. The contaminated water flows through a first cell sandwiched between two cells through which flows recovery solution, which may be plating solution from the bath. The rinse water cell is separated from the cell on one side by a membrane selectively permeable to cations and from the opposite cell by a membrane selectively permeable to anions. An electrical potential is suitably applied between the cells to cause cations and anions in the rinse water to migrate through the appropriate membrane into the adjacent recovery cells. The recovery solution is pumped into the plating bath, thereby recycling the chemicals, while the purified water may be reused for rinsing.
Electrodialysis is typically carried out in an apparatus separate from the plating bath and comprising a plurality of alternating rinse and recovery cells. Electrodes are immersed in opposite end cells for applying the needed electrical potential. The electrodes are connected to an electrical power source distinct from the electroplating power source and generally serve no other productive purpose. In one end cell, the current may be carried by metal ions that plate onto the cathode. However, because of the low contaminant concentrations in the rinse water, electrodialysis produces only a very low electrical current, which is too low to achieve a dense plate of commercially acceptable quality. Rather, the plate is rough or dendritic and may grow so as to damage the adjacent membrane. Also, the anode in the opposite end cell tends to deteriorate. Thus, periodic replacement of the electrodes is required to maintain satisfactory performance of the electrodialysis apparatus.
Therefore, it is an object of this invention to provide a method for electrodialytically recovering dissolved ionic plating chemicals from contaminated rinse water for use in a plating bath, which method is adapted to be carried out in-situ within an electroplating bath concurrent with metal-plating operations that produce a desired, dense, high quality plate. The method utilizes the electrodes and electrical current of the plating operations to effectuate electrodialysis and recover the chemicals directly into the bath, thus eliminating the cost and maintenance required for nonproductive electrodes and an additional power supply used solely for electrodialysis, as well as for a pump to transport recovered chemicals to the bath. Furthermore, the method achieves a suitably high current density at the cathode during plating despite on-going electrodialysis to produce the desired plate and does not foul the electrodes, contaminate the bath or otherwise significantly interfere with electroplating operations.